Sept. 11, 2016: President Obama at a Pentagon ceremony to commemorate the 15th anniversary of the 9/11 terrorist attacks, Arlington, Va. (AP)

President Obama on Sunday paid tribute to the victims and survivors of the 9/11 terror attacks, saying on the 15th anniversary of the strikes that America is stronger but still faces “evolving” and potentially “deadly” terror attacks.

“We remember and we will never forget the nearly 3,000 beautiful lives taken from us so cruelly,” Obama said from the Pentagon, where 184 people were killed.

“We honor the courage of those who put themselves in harm’s way to save people they never knew. … Fifteen years may seem like a long time. But for the families who lost a piece of their heart that day, I image it can seem like just yesterday. … And yet you, the survivors and families of 9/11, your steadfast love and faithfulness has been an inspiration to me and to our entire country.”

The nearly 3,000 people were killed when Al Qaeda hijackers took control of four U.S. passenger jets on Sept. 11, 2001, slamming one into the Pentagon and two into the World Trade Center towers in New York City. (Passengers in the fourth plane overtook the hijackers and took down the plane in Shanksville, Pa.)

Obama said terror threats in recent years have “evolved,” echoing what Homeland Security Secretary Jeh Johnson earlier told “Fox News Sunday” about the country now being safer from another large-scale attack like 9/11, though facing a greater threat from lone-wolf terrorists, inspired by a radical form of Islam.

“Fifteen years into this fight, the threat has evolved,” said Obama, in his final 9/11 memorial speech as president. “With our stronger defenses, terrorists often attempt attacks on a small yet still deadly scale. … Groups like Al Qaeda, like ISIL know they will never be able to defeat a nation as great and as strong as America. So instead they try to terrorize in the hopes that they can still get enough fear that we turn on each other."

He suggested the answer is unity among Americans, in an election year in which immigration -- legal and illegal -- has become a major issue.

"That's why it is so important today that we reaffirm our character as a nation of people drawn from every corner of the world, every color, every religion, every background," Obama said.

He also mentioned several such attacks in recent years -- including the Boston Marathon bombing and the incident in December 2015 in which a husband-wife team in San Bernardino, Calif., who were apparently “self-radicalized,” fatally shot 14 people at a holiday party.

Also on Sunday, Secretary of State John Kerry began his memorial address by remembering the four Americans killed in a Sept. 11, 2011, terror attack on a U.S. outpost in Benghazi, Libya.

He mentioned by name all four -- U.S. Ambassador Chris Stevens, Glen Doherty, Sean Smith and Tyrone Woods -- and called them “dedicated professionals who sought only to serve our country and to help friends in a strife-torn land to live in security and peace.”

The fatal attacks -- as well as revelations about inadequate security and initial attempts to not characterize the incident as terror related -- have been a problem for the Obama administration and Kerry’s predecessor as secretary of state, Democratic presidential nominee Hillary Clinton.

“For us, as a nation, the hours and days after September 11, 2001, remain seared into our minds,” Kerry also said. “With partners on every continent, we are helping to defeat terrorists. … As the American people each day demonstrate, we will neither be intimidated by terrorists, nor draw back from the business of the world, nor retreat from our democratic values."